This invention relates to a method for producing a luminescent screen, as for a cathode-ray tube, which screen exhibits markedly improved adherence to its support during the processing thereof.
In preparing a luminescent screen by the slurry-direct photographic process, as described, for example, in U.S. Pat. No. 3,406,068 to H. B. Law, a glass support is coated with an aqueous slurry comprising a photosensitizable binder (photobinder), a photosensitizer therefor, and particles of phosphor material. Then the coating is dried and then exposed to a light pattern, as by exposure through an apertured mask, to produce regions of greater and regions of lesser solubility in the coating. The coating is then developed through removing the coating regions of greater solubility as by spraying and/or flushing the coating with water or aqueous solutions under pressure. The adherence of the retained less-soluble regions of the coating to the glass surface is important and particularly critical during the development step. The loss of even a small part of the less-soluble regions, which should constitute portions of the screen, requires the screen to be scrapped.
It is known that the adherence of the coating to a clean glass surface can be improved by applying to the glass surface a very thin precoating of a water-soluble polymeric material prior to applying the coating. See, for example, Canadian patent No. 602,838 and U.S. Pat. No. 3,481,733. In a typical process, the surface of a glass panel is washed with an aqueous ammonium bifluoride composition, rinsed with deionized water, then rinsed with a dilute solution of polyvinyl alcohol of about 0.2 to 0.5 weight percent concentration and then dried. It is theorized that a very thin, perhaps monomolecular, precoating of polyvinyl alcohol remains on the glass surface, which precoating improves the adherence of a subsequently applied phosphor coating comprising a dichromate-sensitized polyvinyl alcohol and phosphor particles and having a weight of about 1 to 3 mg phosphor/cm.sup.2. It is not apparent why such a thin precoating improves the adherence of the subsequently applied phosphor coating. However, the improvement is important and the precoating is widely used in the manufacture of screens for color television picture tubes.
When heavier screen weights in the range of 4 to 6 milligrams phosphor per square centimeter (mg/cm.sup.2) are desired, further improvements in adherence are required. Polyvinyl-alcohol precoatings in any weight are ineffective for this purpose. However, an improvement in adhering heavier phosphor coatings can be achieved by employing a heavy precoating of light-exposed dichromate-sensitized polyvinyl alcohol or other photosensitive organic colloid with or without phosphor particles present. Such technique, however, requires the application of a photosensitive material and the subsequent controlled light exposure of the precoating and development to provide the required uniform improvement in adherence which is required. Besides the extra processing steps, this procedure is undesirable in color television picture tubes because some dichromate photosensitizer remains in the precoating and eventually leaves a residue which optically absorbs some of the luminescence from the screen. The novel method employs a thick adherent precoating which is adequate for adhering thicker phosphor layers but which is not photosensitive, does not require exposure to light, and does not leave a light-absorbing residue.